A major industrial problem involves the development of efficient methods for reducing the concentration of air pollutants, such as carbon monoxide, sulfur oxides and nitrogen oxides in waste gas streams which result from the processing and combustion of sulfur, carbon and nitrogen containing fuels. The discharge of these waste gas streams into the atmosphere is environmentally undesirable at the sulfur oxide, carbon monoxide and nitrogen oxide concentrations that are frequently encountered in conventional operations. The regeneration of cracking catalyst, which has been deactivated by coke deposits in the catalytic cracking of sulfur and nitrogen containing hydrocarbon feedstocks, is a typical example of a process which can result in a waste gas stream containing relatively high levels of carbon monoxide, sulfur and nitrogen oxides.
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. In fluidized catalytic cracking (FCC) processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
In the catalytic cracking of hydrocarbons, some nonvolatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons and generally contains from about 4 to about 10 weight percent hydrogen. When the hydrocarbon feedstock contains organic sulfur and nitrogen compounds, the coke also contains sulfur and nitrogen. As coke accumulates on the cracking catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stocks diminishes. Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. This deactivated catalyst is conveyed to a stripping zone where volatile deposits are removed with an inert gas at elevated temperatures. The catalyst particles are then reactivated to essentially their original capabilities by substantial removal of the coke deposits in a suitable regeneration process. Regenerated catalyst is then continuously returned to the reaction zone to repeat the cycle.
Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surfaces with an oxygen containing gas such as air. The combustion of these coke deposits can be regarded, in a simplified manner, as the oxidation of carbon and the products are carbon monoxide and carbon dioxide.
High residual concentrations of carbon monoxide in flue gases from regenerators have been a problem since the inception of catalytic cracking processes. The evolution of FCC has resulted in the use of increasingly high temperatures in FCC regenerators in order to achieve the required low carbon levels in the regenerated catalysts. Typically, present day regenerators now operate at temperatures in the range of about 1100° F. to 1400° F. when no promoter is used and result in flue gases having a CO2/CO ratio in the range of 36 or higher, in a full burn unit to 0.5. The oxidation of carbon monoxide is highly exothermic and can result in so-called “carbon monoxide afterburning” which can take place in the dilute catalyst phase, in the cyclones or in the flue gas lines. Afterburning has caused significant damage to plant equipment. On the other hand, unburned carbon monoxide in atmosphere-vented flue gases represents a loss of fuel value and is ecologically undesirable.
Restrictions on the amount of carbon monoxide, which can be exhausted into the atmosphere and the process advantages resulting from more complete oxidation of carbon monoxide, have stimulated several approaches to the provision of means for achieving complete combustion of carbon monoxide in the regenerator.
Among the procedures suggested for use in obtaining complete carbon monoxide combustion in an FCC regeneration have been: (1) increasing the amount of oxygen introduced into the regenerator relative to standard regeneration; and either (2) increasing the average operating temperature in the regenerator or (3) including various carbon monoxide oxidation promoters in the cracking catalyst to promote carbon monoxide burning. Various solutions have also been suggested for the problem of afterburning of carbon monoxide, such as addition of extraneous combustibles or use of water or heat-accepting solids to absorb the heat of combustion of carbon monoxide.
Specific examples of treatments applied to regeneration operated in the complete combustion mode include the addition of a CO combustion promoter metal to the catalyst or to the regenerator. For example, U.S. Pat. No. 2,647,860 proposed adding 0.1 to 1 weight percent chromic oxide to a cracking catalyst to promote combustion of CO. U.S. Pat. No. 3,808,121 taught using relatively large-sized particles containing CO combustion-promoting metal into a regenerator. The small-sized catalyst is cycled between the cracking reactor and the catalyst regenerator while the combustion-promoting particles remain in the regenerator. Also, U.S. Pat. Nos. 4,072,600 and 4,093,535 teach the use of Pt, Pd, Ir, Rh, Os, Ru, and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory to promote CO combustion in a complete burn unit.
The use of precious metals to catalyze oxidation of carbon monoxide in the regenerators of FCC units has gained broad commercial acceptance. Some of the history of this development is set forth in U.S. Pat. No. 4,171,286 and U.S. Pat. No. 4,222,856. In the earlier stages of the development, the precious metal was deposited on the particles of cracking catalyst. Present practice is generally to supply a promoter in the form of solid fluidizable particles containing a precious metal, such particles being physically separate from the particles of cracking catalyst. The precious metal or compound thereof, is supported on particles of suitable carrier material and the promoter particles are usually introduced into the regenerator separately from the particles of cracking catalyst. The particles of promoter are not removed from the system as fines and are cocirculated with cracking catalyst particles during the cracking/stripping/regeneration cycles. Judgment of the CO combustion efficiency of a promoter is done by the ability to control the difference in temperature, delta T, between the (hotter) dilute phase, cyclones or flue gas line, and the dense phase. Most FCC units now use a Pt CO combustion promoter. While the use of combustion promoters such as platinum reduce CO emissions, such reduction in CO emissions is usually accompanied by an increase in nitrogen oxides (NOx) in the regenerator flue gas.
Promoter products used on a commercial basis in FCC units include calcined spray dried porous microspheres of kaolin clay impregnated with a small amount (e.g., 100 to 1500 ppm) of platinum. Reference is made to U.S. Pat. No. 4,171,286 (supra). Most commercially used promoters are obtained by impregnating a source of platinum on microspheres of high purity porous alumina, typically gamma alumina. The selection of platinum as the precious metal in various commercial products appears to reflect a preference for this metal that is consistent with prior art disclosures that platinum is the most effective group VIII metal for carbon monoxide oxidation promotion in FCC regenerators. See, for example, FIG. 3 in U.S. Pat. No. 4,107,032 and the same figure in U.S. Pat. No. 4,350,614. The figure illustrates the effect of increasing the concentration of various species of precious metal promoters from 0.5 to 10 ppm on CO2/CO ratio.
U.S. Pat. No. 4,608,357 teaches that palladium is unusually effective in promoting the oxidation of carbon monoxide to carbon dioxide under conditions such as those that prevail in the regenerators of FCC units when the palladium is supported on particles of a specific form of silica-alumina, namely leached mullite. The palladium may be the sole catalytically active metal component of the promoter or it may be mixed with other metals such as platinum.
U.S. Pat. Nos. 5,164,072 and 5,110,780, relate to an FCC CO promoter having Pt on La-stabilized alumina, preferably about 4-8 weight percent La2O3. It is disclosed that ceria “must be excluded.” At col. 3, it is disclosed that “In the presence of an adequate amount of La2O3, say about 6-8 percent, 2 percent Ce is useless. It is actually harmful if the La2O3 is less.” In an illustrative example '072 and '780 demonstrates an adverse effect of 8% Ce on CO promotion of platinum supported on a gamma alumina and a positive effect of La.
When sulfur and nitrogen containing feedstocks are utilized in catalytic cracking process, the coke deposited on the catalyst contains sulfur and nitrogen. During regeneration of coked deactivated catalyst, the coke is burned from the catalyst surface that then results in the conversion of a portion of the sulfur and nitrogen to sulfur oxides and nitrogen oxides, respectively.
Unfortunately, the more active combustion promoters such as platinum and palladium also serve to promote the formation of nitrogen oxides in the regeneration zone. It has been reported that the use of prior art CO promoters can cause a dramatic increase (e.g. >300%) in NOx. It is difficult in a catalyst regenerator to completely burn coke and CO without increasing the NOx content of the regenerator flue gas. Since the discharge of nitrogen oxides into the atmosphere is environmentally undesirable, the use of these promoters has the effect of substituting one undesirable emission for another. Many jurisdictions restrict the amount of NOx that can be in a flue gas stream discharged to the atmosphere. In response to environmental concerns, much effort has been spent on finding ways to reduce NOx emissions.
Various approaches have been used to either reduce the formation of NOx or treat them after they are formed. Most typically, additives have been used either as an integral part of the FCC catalyst particles or as separate particles in admixture with the FCC catalyst.
Various additives have been developed that will carry out CO promotion while controlling NOx emissions.
U.S. Pat. Nos. 4,350,614, 4,072,600 and 4,088,568 mention rare earth addition to Pt based CO promoters. An example is 4% REO that shows some advantage. There is no teaching of any effect of REO on decreasing NOx emissions from the FCCU.
U.S. Pat. No. 4,199,435 teaches a combustion promoter selected from the Pt, Pd, Ir, Os, Ru, Rh, Re and copper on an inorganic support.
U.S. Pat. No. 4,290,878 teaches a Pt—Ir and Pt—Rh bimetallic promoter that reduces NOx compared to conventional Pt promoter.
U.S. Pat. No. 4,300,997 teaches the use of a Pd—Ru promoter for oxidation of CO that does not cause excessive NOx formation.
U.S. Pat. No. 4,544,645 describes a bimetallic of Pd with every other Group VIII metal but Ru.
U.S. Pat. Nos. 6,165,933 and 6,358,881 to W. R. Grace describe compositions comprising a component containing (i) an acidic oxide support, (ii) an alkali metal and/or alkaline earth metal or mixtures thereof, (iii) a transition metal oxide having oxygen storage capability, and (iv) palladium; to promote CO combustion in FCC processes while minimizing the formation of NOx.
U.S. Pat. No. 6,117,813 teaches a CO promoter consisting of a Group VIII transition metal oxide, Group IIIB transition metal oxide and Group IIA metal oxide.
There is still a need, however, for improved CO oxidation promoters having NOx emission control in FCC processes.